During paper manufacture, titanium dioxide pigments are added to cellulose fibers for imparting whiteness, opacity, and/or UV protection to the finished product, which generally contains 4%-5% titanium dioxide. Current practice makes inefficient use of the titanium dioxide, which is an expensive paper component. The titanium dioxide particles are not retained well on the cellulose fibers, requiring flocculation treatment with large excesses of titanium dioxide leading to much wasted pigment. Recycling of titanium dioxide from this process increases production costs. In addition this treatment entraps titanium dioxide particles thereby filling cellulose to cellulose bonding sites and weakening the overall paper strength, such that wet strength resins must be added to restore the strength of the paper product.
A particular application demanding high titanium dioxide pigment retention and high quality of optical properties is use in paper incorporated into paper laminates, also called decorative films, for decorative applications. Paper laminates are in general well-known in the art, being suitable for a variety of uses including table and desk tops, countertops, wall panels, floor surfacing, tableware and the like. Paper laminates have such a wide variety of uses because they can be made to be extremely durable, and can be also made to resemble (both in appearance and texture) a wide variety of construction materials, including wood, stone, marble and tile, and can be decorated to carry images and colors.
Typically, the paper laminates are made from papers by impregnating the papers with resins of various kinds, assembling several layers of one or more types of laminate papers, and consolidating the assembly into a unitary core structure while converting the resin to a cured state. The type of resin and laminate paper used, and composition of the final assembly, are generally dictated by the end use of the laminate.
Decorative paper laminates can be made by utilizing a decorated paper layer as the upper paper layer in the unitary core structure. The remainder of the core structure typically comprises various support paper layers, and may include one or more highly-opaque intermediate layers between the decorative and support layers so that the appearance of the support layers does not adversely impact the appearance of the decorative layer. Various papers in the paper layers may contain pigments as well as additives such as wet-strength, retention, sizing (internal and surface) and fixing agents as required to achieve the desired end properties of the paper. In particular, the finished decorative paper in the laminate contains approximately 10% to 65% titanium dioxide pigment to provide opacity, eliminating any show-through of the substrate for the laminate. Thus high retention of titanium dioxide pigment in papers used in decorative paper laminates is important for the manufacture of a decorative paper laminate product.
U.S. Pat. No. 3,503,772 discloses improvement in the paper retention and UV stability properties of titanium dioxide by coating titanium dioxide particles with SiO2 and ZnO, then dry milling in the presence of a water-soluble alkali metal carboxymethylcellulose.
GB 1025960 discloses the use of using titanium dioxide particles coated with chemically aminated polysaccharides to aid retention of titanium dioxide on cellulose for low pigmented paper.